1. Field of the Invention
The present invention relates to a process for manufacturing a contact material for vacuum circuit breakers, and in particular to a manufacturing process by which a contact material having a lower restriking frequency can be obtained.
2. Description of the Prior Art
Contact materials for vacuum circuit breakers are basically required to have excellent material characteristics such as a weld resistant property, an ability to withstand voltages applied to the contact materials and an ability to completely break current without restriking and igniting. However, it is difficult to satisfactorily meet all of the requirements, because the physical nature of these requirements contradict one another. As a result, when the contact material is being manufactured, priority, as according to the particular circuit breaker, is given to the most important requirement for application to the circuit breaker at the sacrifice of the other requirements.
An example of a prior art electrode contact for a vacuum circuit breaker having a large capacity can be found in Japanese Patent Publication No. 41-12131, which discloses a copper alloy containing a weld inhibitor ingredient such as bismuth, tellurium, tin and the like at an amount of less than 5% by weight. In this connection, since the vacuum circuit breakers have advantages over other circuit breakers in that their bodies are compact and light weight and can be adapted to the environment with less repair, etc., the possible applications of vacuum circuit breakers have been increasing year by year. This has led to a drastic increase in demand for vacuum circuit breakers not only for the popular conventional use in which they are used with circuits at a voltage of less than 36 KV, but also for special uses such as making and breaking a condenser circuit. Therefore, vacuum circuit breakers are now required to have a higher voltage-withstanding property. Unfortunately, the above-mentioned prior art materials are not able to satisfy such recent requirements.
One of the reasons for regarding the voltage withstanding property of the prior art material as being insufficient for use with high-voltages stems from it's tendency to cause restriking and reignition phenomena. Accordingly, in order to improve the reliability of the contact products, it is recognized that these phenomena must be understood. However, the restriking and reignition phenomena have not yet been traced to a direct origin, nor has any technique been developed for preventing the phenomena.
Due to current demands, the contact material is required to have a higher voltage-withstanding property and a lower restriking frequency for use with higher voltages as mentioned above. To improve the voltage withstanding property of the contact material and decrease the restriking frequency, it is preferred to decrease the content of the weld inhibitor ingredient as much as possible and prevent the weld inhibitor ingredient from being particularly disuniformly concentrated in the material. The weld inhibitor makes the material brittle and is therefore disadvantageous to the pressure resistance of the material, the decrease of gaseous impurities and pin holes in the material, and the strengthening of the contact alloy itself. With respect to these points, the above-mentioned prior art Cu-Bi material does not satisfactorily meet the requirements of recent trend.
On the other hand, a sintered copper-tungsten alloy material and a copper-tungsten carbide (Cu-WC) alloy material are known as another conventional contact materials. Despite the fact that these sintered contact alloy materials arc rather remarkable in their voltage withstanding property, they tend to contain pores due to the manufacturing method, thereby causing the sintered material to actively produce thermoelectrons. As a result, this conventional alloy has the defect of easily causing the restriking phenomenon.
Moreover, in the field of vacuum circuit breakers for use with high voltages and heavy currents, a copper-chromium alloy material is utilized for the contact material. In this alloy material, since the difference in the vapour pressures of the component elements is not as large as that of the other materials, this alloy has an advantage in that its properties can be expected to be stable. Accordingly, the characteristics of this contact alloy material may be useful depending upon the manner in which it is used.
The above-mentioned Cu-Cr alloy contact material is generally manufactured according to the process which is disclosed, for example, in Japanese Patent Publication No. 59-30761 as comprising the steps of: mixing Cr powder with a small amount of Cu powder; pouring the mixed powder into a die and pressing the powder slightly to form a compact; taking the compact out from the die and sintering the compact in a vacuum to form a Cr skeleton; and finally infiltrating a Cu component into the Cr skeleton. Moreover, another manufacturing process is disclosed in Laid-Open Japanese Patent Application No.(Kohkai) 59-25903. This reference discloses a method in which a Cu pellet is mounted on a Cr powder being poured into a die. The pellet then infiltrates into the Cr powder after air is removed from the Cr powder and Cu pellet under reduced pressure. Furthermore, the Cu-Cr alloy material can be obtained by a solid-phase sintering method in which Cu powder and Cr powder are first mixed at a desired content and compacted. Then the compact is sintered at a temperature equal to or less than the melting point of the Cu element.
However, in the above-described processes by which the alloy material is generally obtained using a powder metallurgical technique, proper management of the raw material powder, and sintering and infiltration techniques have not yet been developed to adequately cope with the generation of the restriking phenomenon. Therefore, they cannot give satisfactory results with respect to the restriking frequency.